The invention relates to a steering gear and particularly to a steering gear for a power steering system of a motor vehicle.
In the case of most of the motor vehicles, power steering systems are installed which generate a supporting torque while steering and therefore reduce the steering torque, which has to be applied by the driver to the steering column in order to cause the steering lock of the wheels.
The known power steering systems are based on a steering gear that translates the driving power of a hydraulic or electric drive and transfers the same to the steering column. Such steering gears are normally constructed in the form of a screw rolling gear and particularly as a helical drive gear or worm gear, i.e. these comprise a gearwheel that is directly or indirectly connected to the steering link as well as pinion shaft which meshes with the gearwheel and is driven by the auxiliary drive. The gearwheel as well as the pinion shaft is frequently made from plastic.
Gear backlash has been shown to be a problem in such steering gears, which occurs due to component tolerances, different heat expansion of the components and as a result of wear, respectively setting processes of the materials used for the gearwheel and the pinion shaft during the service life of the steering gear. In particular in the case of so-called change of steering i.e. the directly consecutive steering with variable steering lock, such a gear backlash generates undesirable noises, which result from the alternating abutment of opposing flanks of the teeth of the pinion shaft and gear wheel.
It is known that this gear backlash can be eliminated in that the pinion shaft can be mounted so as to pivot about an axis, which runs perpendicular to the longitudinal axis of the pinion shaft and spaced apart from the meshing engagement of pinion shaft and gearwheel and is pressed against the gearwheel by means of one or a plurality of spring elements. The pivotability of the pinion shaft is thereby integrated into one of the two bearings, via which the pinion shaft is mounted at the end. This bearing is referred to as a locating bearing. The bearing in the region of the other end is the designed with play (so-called “floating bearing”) in order to facilitate the deflection caused by the pivoting movement. The locating bearing is generally provided on the drive side, while the floating bearing is provided at the free end of the pinion shaft.
A corresponding steering system is, for example, known from the WIPO patent application WO 2011/073089 A1. Provision is thereby made for the roller bearing, which accommodates the pinion shaft in the region of the locating bearing, to be mounted on the outside in a two-piece bearing bush. The bearing bush comprises an inner ring, which substantially accommodates the roller bearing free of play and an outer ring, which is substantially held free of play, wherein the outer and the inner ring are connected via narrow webs, which are elastically twisted when the outer ring is rotated towards the inner ring. The webs define not only the pivot axis, but these also act as spring elements, by means of which the pinion shaft is loaded against the gearwheel. To this end, a mounting of the pivot ring takes place with already twisted webs.
It is also known to integrate the one or plurality of spring elements for pressing the pinion shaft against the gearwheel into the floating bearing as this is, for example, disclosed in the German patent application DE 10 2007 055 814 A1.
In order to be able to compensate for the tolerances of the components of the steering gear due to manufacture as well as for the thermal expansions and for the effects of wear or respectively setting processes as a result of a use of the steering gear, it is necessary in the case of the known steering gears to provide a relatively large spring bias for the pinion shaft and thus prestressing of the spring elements. This has however the disadvantage that, when the steering gear is new if the spring bias has not yet been reduced as a result of a compensation of wear and setting effects, the strength with which the pinion shaft is pressed against the gearwheel is relatively large, which is connected with a correspondingly large gear friction and thus disadvantages in efficiency. This applies at least to those steering gears which do not fully utilize the admissible tolerances in total and thus for the spring bias provided to be oversized.